A decanter centrifuge comprises a hollow drum cylindrical/conical cross-section rotatably supported by bearings and having a helical conveyor therein, rotatably supported by bearings relative to the drum. These centrifuges are used primarily where the mixture to be treated has a relatively large content of solids, or where the content of particles in a mixture varies greatly or varies greatly in size. They also find use in applications where relatively large volumes of liquid are treated. They provide a satisfactory separation despite large flows per unit time. Decanter centrifuges are used to separate solids from a liquid mixture in which the solids in the form of particles are suspended in a liquid having a density either greater or less than that of the particles. The major use of decanter centrifuges is in the treatment of sludge from sewage treatment plants (U.S. Pat. No. 6,123,656 and U.S. Pat. No. 6,537,191), though they have also found use in oil recovery systems (U.S. Pat. No. 6,214,236), and for defatting meats and other food products (U.S. Pat. No. 5,944,597).
Decanter centrifuges have also been used to concentrate dehydrated emulsions of water soluble polymers in a water-in-oil polymer emulsion (U.S. Pat. No. 6,110,995).
U.S. Pat. No. 2,718,452 describes a process for forming polytetrafluoroethylene organosols, in which the layer containing the organosol is decanted, and then centrifuged to remove occluded water. U.S. Pat. No. 3,536,683 described a process for isolating fluorine-containing polymers involving agglomerating the polymer by adding electrolyte, followed by separation of the agglomerates from the supernatant liquid in a bowl centrifuge, washing and re-centrifuging prior to drying. The washing helps remove electrolyte, which would be an undesired impurity in the final dried products, but the washing step adds complexity to the process, and adds additional water to the polymer, making drying more difficult.
It is known to remove water or aqueous solution from polymer latex emulsion by various means. Some examples are thickener, filter, etc. In a current commercial practice fluoropolymer latex is processed through an agitated vessel(s) with or without latex washing, latex thickening and finally dried. Optionally, latex washing can be conducted in an agitated vessel or after the agitated vessel as a separate unit operation. The conventional thickening operation has its limitation on the dewater efficiency and overall yield, and the dewatered polymer latex stream still contains a large amount of water, requiring added processing time and energy for its removal. Furthermore, the carryover in the aqueous discharge can be high and reduces the overall process yield. Such fluoropolymer slurry typically floats to the top of the thickening device since its density is less than that of water leading to a low thickening efficiency. In some fluoropolymer operations, reverse osmosis has been mentioned for thickening, in which the operating difficulties, such as membrane fouling, is known.
There is a need for a thickening or dewatering process that operates at higher throughput, higher efficiency, removes more solvent or water (produces a higher polymer solids) and generates higher yield than current processes. The need is especially great for emulsion or aqueous suspension polymers produced at a relatively low solids level, and therefore containing large amounts of solvent or water.
Surprisingly it has been found that the use of a decanter centrifuge in the processing of aqueous polymer emulsions and suspensions, such as a fluoropolymer latex, offers a high throughput and improves the thickening efficiency while providing a consistent solid content in the dewatered polymer latex. Optionally, a horizontal decanter centrifuge is used. The decanter centrifuge can be used to process any polymer suspension, solution or mixture having significantly large particles (which can be masses consisting of more than one polymer particle, such as agglomerates or coagulants), where there is a density different (either more or less) between the polymer particles and the continuous aqueous phase. In the case of fluropolymers, it was found that the polymer agglomerates were less dense than the aqueous phase. Polymer agglomerates are typically derived from the combination or aggregation of colloidal polymer particles dispersed or suspended in a liquid into clusters or matrixes.
The decanter result for the fluoropolymers is unexpected since conventional operation of a decanter centrifuge typically involves a solid component that is denser than the liquid component on the theory that the denser phase travels faster than the less dense phase in a centrifugal field. In the process of the invention the polymer-agglomerates are less dense than the continuous (solvent) phase. Depending on the operating parameters of the decanter centrifuge, the polymer agglomerates can be discharged as the higher solids floater. The solid content of the solid discharge can be either increased or decreased by adjusting operating parameters of the decanter centrifuge.